The installation of electrical devices, such as circuit breakers, in switchboard and panelboard apparatus is generally of two types. In one type of installation, circuit breakers are essentially stationarily mounted to structural members or frame of the switchboard, typically with terminal electrical connection effected via bolted joints. Alternatively, circuit breakers may be physically supported solely by the switchboard buswork via these bolted terminal joints. In such stationary installations, inspection and maintenance of the circuit breaker is difficult and quite hazardous if attempted while the switchboard is live. In some situations, the hazards must be assumed, since de-energization of the switchboard and thus interruption of electrical service to all of the branch circuits in the switchboard cannot be tolerated. To reduce the hazards of working with live switchboards, devices are provided with plug-in or stab-type primary disconnect contacts which do not require direct contact and manipulation to effect their connection and disconnection with mating disconnect contacts of the switchboard. Thus, the device, e.g., circuit breaker, can be plugged in and unplugged from a live switchboard in relative safety.
In higher current applications, the circuit breakers are physically large and quite heavy, thus rendering the bodily movement of the circuit breaker necessary to electrically connect and disconnect it from the switchboard cumbersome if not impossible, unless mechanical assistance is afforded. Not only does the weight and bulk of the circuit breaker become difficult for the electrician to handle, but, as the current ratings increase, the contact pressures of the primary disconnect contacts become extremely difficult, if not impossible to overcome. To surmount these problems, so-called "drawout" apparatus has been resorted to for both supporting the circuit breaker and affording mechanical assistance in overcoming the extreme contact pressures of the disconnect contacts.
Drawout apparatus are typically designed to support the electrical device for racking movement between an extended position well out of an enclosure compartment and an engaged position within the compartment where the load current carrying primary disconnect contacts of the device and enclosure are fully engaged. Since electrical devices, such as circuit breakers for industrial applications, are typically equipped with a variety of accessorial functions served by external auxiliary circuits, provisions must be made for making and breaking these auxiliary circuits as the circuit breaker is moved between its extended and engaged positions. To this end, so-called mating secondary disconnect contacts are mounted with the circuit breaker and the enclosure. These secondary contacts are engaged while the circuit breaker is in the engaged position and become disengaged at some point during movement of the breaker out to the disengaged position. It is common practice to provide a test position for the circuit breaker intermediate its engaged and extended positions where the primary contacts are separated but the secondary contacts are still engaged. In this test position, the accessorial functions can be thoroughly tested in safety while the breaker is "dead".
During movement between the test and engaged positions, it is imperative that the circuit breaker internal contacts be open, otherwise violent arcing will typically occur as the primary contacts begin engaging while the circuit breaker is moved toward its engaged position and as the primary contacts begin disengaging and ultimately separate while the breaker is en route from the engaged to the test position. Such violent arcing will invariably do considerable damage to the primary contacts and presents an extremely hazardous situation for both personnel and equipment.
It is accordingly an object of the present invention to provide drawout apparatus which is equipped with an interlock operable to insure that the circuit breaker internal contacts are open while the circuit breaker is being moved between its test and engaged positions.
An additional object is to provide drawout apparatus of the above character, wherein the interlock is automatically operable to permit closure of the breaker contacts while the breaker is in its test position and its fully engaged position, and to inhibit breaker contact closure while the circuit breaker is anywhere between these two positions.
Another object of the invention is to provide drawout apparatus of the above character wherein with the circuit breaker in either its test or engaged position and its contacts closed, the interlock operates to initiate tripping of the circuit breaker automatically incident to initial movement of the breaker away from one of these positions toward the other.
A further object is to provide a trip interlock of the above character which is reliable and fool-proof in operation, and inexpensive to manufacture.
Other objects of the present invention will in part be obvious and in part appear hereinafter.